Interobserver Agreement of a Probabilistic Approach to Reporting Breast Fine-Needle Aspirations on ThinPrep Bradley Gornstein, M.D., 1 Timothy Jacobs, M.D., 1 Yvan Be ´ dard, M.D., Ph.D., 2 Charles Biscotti, M.D., 3 Barbara Ducatman, M.D., 4 Lester Layfield, M.D., 5 Grace McKee, M.D., 6 Nour Sneige, M.D., 7 and Helen Wang, M.D., Dr.P.H. 1 * We have previously demonstrated the accuracy and reproducibil- ity of a probabilistic/categorical approach for reporting breast fine-needle aspiration (FNA). However, the interobserver agree- ment in the application of this approach has not been assessed. Twenty breast FNA cases (each on one ThinPrep slide) were pulled from the cytology files of Beth Israel Deaconess Medical Center. The cases included benign epithelial proliferative lesions (6), DCIS (4), and infiltrating carcinoma (10), as shown by sub- sequent histology. Six pathologists with 14 –25 yr of experience in interpreting breast FNA and 0 – 8 yr of experience with ThinPrep preparations rendered diagnoses according to the probabilistic approach. The kappa statistic for the unremarkable/proliferative, atypical, suspicious, and positive categories were 0.64, 0.08, 0.43, and 0.75, respectively (P  0.001 for all except for the atypical category [P = 0.09]). Spearman’s rho correlating the individual pathologist’s diagnosis and the histologic diagnosis ranged from 0.51 (P = 0.02) to 0.78 (P  0.0001). This was not correlated with the pathologists’ years of experience interpreting breast FNA (P = 1.0) or with their years using ThinPrep preparations for breast FNA (P = 0.96). In conclusion, the interobserver agreement was excellent for the positive category in the probabilistic approach, poor for the atypical category, and fair to good for the other categories. The specific level of experience interpreting breast FNA or using ThinPrep among experienced pathologists did not seem to influence their accuracy in reporting the cases in our study. Diagn. Cytopathol. 2004;30:389 –395. © 2004 Wiley-Liss, Inc. Key Words: breast; fine-needle aspiration; interobserver agreement Breast fine-needle aspiration (FNA) has long been recog- nized to be a cost-effective method for preoperative diag- nosis of breast cancer. 1 It has also been recognized that low-grade in situ and invasive carcinomas, atypical ductal epithelial lesions, and fibroadenomas are diagnostic pit- falls 2–7 that can compromise the sensitivity and specificity of this otherwise highly accurate diagnostic test. 8 With the trend toward evaluation of radiologically detected nonpal- pable lesions, such “borderline” lesions are encountered with increasing frequency. 9,10 Many authors have recog- nized this issue and have proposed various categorical ap- proaches to reporting breast FNA. 11–15 The approach de- scribed by Wang and Ducatman 15 has simple, well-defined cytologic criteria for each category and the categories are ranked according to their probabilities of finding carcinoma on histology. For example, the probabilities of finding car- cinoma on histology for the cases in the positive, suspicious, atypical, and proliferative/unremarkable cytology catego- ries are 100%, 80 –90%, 20 –30%, and 5%, respectively. It has also been shown that cytopathologists with varying degrees of experience can easily apply these criteria to yield reproducible probabilities of carcinoma associated with each category. 16 We conducted this study to further test the interobserver agreement of these criteria among experi- enced breast cytopathologists for a selected group of biop- 1 Department of Pathology, Beth Israel Deaconess Medical Center, Bos- ton, Massachusetts 2 Department of Pathology, Mount Sinai Hospital and University of Toronto, Toronto, ON, Canada 3 Department of Pathology, The Cleveland Clinic Foundation, Cleveland, Ohio 4 Department of Pathology, Robert C. Byrd Health Sciences Center of West Virginia University, Morgantown, West Virginia 5 Department of Pathology, University of Utah School of Medicine, Salt Lake City, Utah 6 Department of Pathology, Massachusetts General Hospital, Boston, Massachusetts 7 Department of Pathology, University of Texas M.D. Anderson Cancer Center, Houston, Texas Current affiliation for Bradley Gornstein: Charlton Memorial Hospital, Fall River, MA. Current affiliation for Timothy Jacobs: Virginia Mason Medical Center, Seattle, WA. * Correspondence to: Helen H. Wang, M.D., Department of Pathology, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, Boston, MA 02215. E-mail: hwang@bidmc.harvard.edu Received 19 May 2003; Accepted 21 November 2003 DOI 10.1002/dc.20041 Published online in Wiley InterScience (www.interscience.wiley.com). © 2004 WILEY-LISS, INC. Diagnostic Cytopathology, Vol 30, No 6 389